Ion implantation has become a standard technique for introducing conductivity-altering impurities into semiconductor wafers. A desired impurity material is ionized in an ion source, the ions are accelerated to form an ion beam of prescribed energy and the ion beam is directed at the surface of the wafer. The energetic ions in the beam penetrate into the bulk of the semiconductor material and are embedded into the crystalline lattice of the semiconductor material to form a region of desired conductivity.
Ion implantation systems usually include an ion source for converting a gas or solid material into a well-defined ion beam. The ion beam is mass analyzed to eliminate undesired ion species, is accelerated to a desired energy and is directed onto a target plane. The beam may be distributed over the target area by beam scanning, by target movement or by a combination of beam scanning and target movement. Examples of prior art ion implanters are disclosed in U.S. Pat. No. 4,276,477 issued Jun. 30, 1981 to Enge; U.S. Pat. No. 4,283,631 issued Aug. 11, 1981 to Turner; U.S. Pat. No. 4,899,059 issued Feb. 6, 1990 to Freytsis et al.; and U.S. Pat. No. 4,922,106 issued May 1, 1990 to Berrian et al.
U.S. Pat. No. 5,350,926 issued Sep. 27, 1994 to White et al. discloses a high current, broad beam ion implanter which employs a high current density ion source, an analyzing magnet to direct a desired species through a resolving slit and an angle corrector magnet to deflect the resultant beam, while rendering it parallel and uniform along its width dimension. A ribbon-shaped beam is delivered to a target, and the target is moved perpendicular to the long dimension of the ribbon beam to distribute the ion beam over the target.
A well-known trend in the semiconductor industry is toward smaller, higher speed devices. In particular, both the lateral dimensions and the depths of features in semiconductor devices are decreasing. State of the art semiconductor devices require junction depths less than 1,000 Angstroms and may eventually require junction depths on the order of 200 Angstroms or less.
The implanted depth of the dopant material is determined, at least in part, by the energy of the ions implanted into the semiconductor wafer. Shallow junctions are obtained with low implant energies. However, ion implanters are typically designed for efficient operation at relatively high implant energies, for example in the range of 20 keV to 400 keV, and may not function efficiently at the energies required for shallow junction implantation. At low implant energies, such as energies of 2 keV and lower, the current delivered to the wafer is much lower than desired and in some cases may be near zero. As a result, extremely long implant times are required to achieve a specified dose, and throughput is adversely affected. Such reduction in throughput increases fabrication cost and is unacceptable to semiconductor device manufacturers.
In one prior art approach to low energy ion implantation, the ion implanter is operated in a drift mode with the accelerator turned off. Ions are extracted from the ion source at low voltage and simply drift from the ion source to the target semiconductor wafer. However, a small ion current is delivered to the wafer because the ion source operates inefficiently at low extraction voltages. In addition, the beam expands as it is transported through the ion implanter, and ions may strike components of the ion implanter along the beamline rather than the target semiconductor wafer.
Another prior art approach utilizes a deceleration electrode in the vicinity of the semiconductor wafer. See, for example, European Patent Application No. 0,451,907 published Oct. 16, 1991. Ions are extracted from the source, are accelerated by a higher voltage and then are decelerated by the deceleration electrode before being implanted into the wafer. This approach also suffers from ion beam expansion and energy contamination in the beam delivered to the wafer. An ion implanter wherein a deceleration electrode is positioned between an ion source and a mass separator is disclosed in European Patent Application No. 0,685,872 published Dec. 6, 1995.
Delivery of low (less than 10 keV) and ultra-low (less than 1 keV) energy, mass resolved, monoenergetic ion beams to a target with currents greater than a few microamperes is difficult. As noted above, space charge effects can produce rapid divergence of the beam envelope, impeding transmission and reducing the ultimate beam current delivered to the target. As further noted above, beams are often transported to the vicinity of the target at higher energies and are then decelerated to the final energy by means of a retarding electric field in order to overcome the beam expansion problem. A consequence of the deceleration is that ions neutralized prior to entering the retarding field region impact the target with the transport energy. These higher energy neutralized ions are implanted into the wafer and are likely to have adverse affects on device performance due to their deeper than desired location below the surface.
For ion implantation, two properties of this higher energy implant, or energy contaminant, are important: the quantity of neutralized high energy ions and their energy at the moment of neutralization. Typical single magnet beamlines require the beam to possess several multiples of the final energy to achieve satisfactory beam currents. Moreover, the necessary species selection and mass resolution of the final implant beam demand a long path from the exit of the magnet to the target, thereby increasing the total number of ions neutralized. Consequently, conventional beamlines designed to produce milliamps of beam current at low and ultra-low energies must contend with an undesired energy contaminant comprising large numbers of neutralized high energy ions.
Known prior art ion implanters are not capable of delivering relatively high current, energy pure ion beams at low and ultra-low energies. Accordingly, there is a need for improved methods and apparatus for low energy ion implantation.